Polyethylene is a prototype polymer with a simple molecular structure, which is widely used in forming articles subject to lightly loaded sliding contacts, such as furniture feet and drawer glides. Polyethylenes can exist in a wide range of states, with vastly different physical properties. For example, low molecular weight polyethylene molecules are gases or liquids; at about 20,000 molecular weight, polyethylene is considered a plastic; and, up to about 500,000 molecular weight, polyethylene is generally considered suitable for injection molding and extrusion. "Very high molecular weight" polyethylene has a molecular weight between about 500,000 and 1,000,000 and "ultrahigh molecular weight polyethylene" (abbreviated UHMWPE) has a molecular weight between about 1,000,000 and 5,000,000, UHMWPE is generally only processable by compression molding and ram extrusion.
In polymeric materials, polyethylene chains can exist simultaneously in any one of three states, namely: an amorphous state containing randomly enterwined molecules; a crystalline state with neatly arranged dense packing of molecules; or, an oriented state produced by partial orientation of molecules during polymer processing. While it is highly desirable to manufacture materials having a given molecular orientation, the results are still largely unpredictable, especially when formulated with additives.
Several polyethylene polymers have been disclosed that successfully incorporate lubricating oils, which may bleed to the surface and perform a lubricating function. Agens (U.S. Pat. No. 3,135,564) formulated such a lubricant from vinyl chloride. Davis (U.S. Pat. No. 3,541,011; U.S. Pat. No. 3,547,819; U.S. Pat. No. 3,729,415 teaches formulation of a lubricant dispensing oil from an UHMWPE sometimes in combination with certain lower molecular weight polyethylenes. Rumierz (U.S. Pat. No. 4,146,487) disclosed similar results by formulating compositions with polymethyl pentene, having a range of molecular weights between 3,000,000 and 5,000,000. Jamison, the inventor of the subject invention, in an earlier patent (U.S. Pat. No. 4,486,319) disclosed that lubricants might be formulated using ionomer polymers. Baile (U.S. Pat. No. 4,239,632) disclosed certain properties of oil-exuding plastics and modification with a solid additive designed to conduct away frictional heat. Jamison in an earlier patent (U.S. Pat. No. 4,915,856) describes a lubricating composition formulated from certain polymers, oils, and both solid and liquid additives. Takeshi et al. (U.S. Pat. No. 5,079,287) disclosed formulations with at least 10.5% of UHMWPE. In general, all the former lubricant compositions using UHMWPE suffer from common disadvantages, namely, that UHMWPE is poorly formable by injection molding or extrusion, and that bleed rates of oil from the lubricant compositions are poorly controlled even when using a standardized, rigidly controlled manufacturing process.
Several attempts have been made in the art to control bleed rate from polyethylene polymers. Ikeda and Kawakita (U.S. Pat. No. 3,779,918) describe polyethylene polymers formulated with oil adsorbed onto either a graphite carrier, a polymer powder such as Hostalen GUR, or a natural fiber such as cotton. The process of forming the latter polymer reportedly fuses the oil and carrier adsorbent into the polymer. A requirement that the carrier have a higher thermal stability than the base resin suggests that the resultant composition remains a two-phase material at all times during; and after molding. Ikeda and Ishikawa (U.S. Pat. No. 3,985,661) similarly disclose formulation of a lubricant composition from carrier adsorbed oil, wherein the oil remains as a second phase in the resultant polymeric material. U.S. Pat. No. 4,829,093 also pertains to the absorption of oil onto a preformed carrier, but does not teach how to control the subsequent release of oil from within polymeric structure. In general, two-phase systems for controlling bleed rates are disadvantageous for the following reasons: first, the resulting polymer is structurally weakened by inclusion of oil and carrier adsorbent; and, second, the bleed rate of the oil from within the polymer-carrier matrix is still poorly controlled.
Aside from the use of polyethylenes, several prior art attempts have been made to formulate other polymeric compositions with controlled bleed rates. Landkamp, U.S. Pat. No. 4,448,700, reportedly formulated a polymeric material of polypropylene or a mixture of polypropylene and nylon as a resin, using a polyol ester as the lubricant. Since the latter disclosure specifies the use of a certain type of synthetic oil, the compositions may not (in practice) be widely applicable. Aboshi et al. (U.S. Pat. No. 4,041,002) reportedly formulated an ethylene-vinyl acetate (EVA) copolymer with an acetal base resin. Since acetal will not dissolve any oil, the oil must be dissolved in the EVA. Matsukawa and Ishioka (U.S. Pat. No. 4,829,093) discuss the use of a microporous styrene-divinyl benzene additive to incorporate oil into a polymeric composition, but oil viscosity must reportedly be decreased with a solvent to prepare the disclosed composition.
Thus, although the desirability has been recognized in the art of polymeric lubricant compositions having a controlled bleed rate of oil, the prior art has not recognized how to formulate and extrude these materials, how to control bleed rate, or how to reduce the rate of release of a lubricating oil from within the structure of a polyethylene.